1. Field of the Invention
The present invention relates to an image display apparatus for displaying an image by utilizing a pixel shift.
2. Related Art Statement
In Japanese Patent Application Laid-open Publication Kokai Hei 4-113308, there is proposed a known image display apparatus, in which an image of one frame is consisting of images of n fields, and a position of the images displayed in respective fields is shifted in a stepwise manner within a pixel pitch in up and down and right and left directions or in up and down, right and left and oblique directions. In this manner, the number of displaying image elements or pixels is equivalently increased by n times. FIG. 61 is a block diagram showing such a known image display apparatus. In front of an image displaying liquid crystal (LC) panel 1, there is arranged an image pixel shift element 4 comprising a polarizing direction controlling LC panel 2 and a birefringent or double refractive plate 3. Then, an original image can be displayed to have the number of pixels which is twice of that of pixels of the image displaying LC panel 1.
In the image display apparatus shown in FIG. 61, an input image signal of one frame is supplied to a divider 5 and is converted into image signals of two fields each corresponding to every other pixels viewed in the horizontal direction. These image signals are stored in frame memories 6 and 7. The image signals stored in the frame memories 6 and 7 are read-out successively under the control of a synchronizing signal generator 8 in such a manner that the image signals are read out alternately for successive fields and are supplied to the image displaying LC panel 1. In synchronism with the reading-out operation, a driving voltage generator 9 applies a given voltage to the polarizing direction controlling LC panel 2.
In case of displaying an image signal stored in the first frame memory 6, a voltage is applied to the polarizing direction controlling LC panel 2 such that an image displayed on the image displaying LC panel 1 is transmitted through the LC panel 2 without rotating a polarizing direction. Then, the image is made incident upon the birefringent element 3 as the ordinary light. Therefore, the image can be seen by a user such that the image is displayed at an ordinary position on the image displaying LC panel 1 as depicted in FIG. 62A. In the other words, no pixel shift occurs. When an image signal stored in the frame memory 7 is displayed, no voltage is applied to the polarizing direction controlling LC panel 2 from the driving voltage generator 9, and thus the polarizing direction of light emanating from the image displaying LC panel 1 is rotated by 90 degrees. Therefore, the light is made incident upon the birefringent element 3 as the extraordinary light. Therefore, the image can be seen by the user such that the image is displayed on the image displaying LC panel 1 at a position which is shifted by a half pixel pitch in the horizontal direction as illustrated in FIG. 62B.
As explained above, in the known image display apparatus shown in FIG. 61, the image signal of a frame is divided into two fields, and these fields are alternately displayed at a high rate. Then, the number of pixels in the horizontal direction can be increased twice as depicted in FIG. 62C. That is to say, the interpolation of pixels in the horizontal direction can be attained and the resolution in the horizontal direction can be increased.
In Japanese Patent Application Laid-open Publication Kokai Hei 7-13163, there is disclosed another known image display apparatus performing the vertical pixel shift. In case of displaying NTSC standard color television signal on an image display device such as a liquid crystal display having a lower resolution in the vertical direction, image signals of odd and even fields are not displayed on the same horizontal lines, but the pixel shift is effected in the vertical direction to improve the resolution in the vertical direction.
FIG. 63 shows another known image display apparatus using the above mentioned pixel shift technique. In front of an image displaying LC panel 1, there are arranged two pixel shift elements 4-1 and 4-2 successively, each of said pixel shift elements comprising a polarizing direction controlling LC panel and a birefringent element. The image displaying LC panel 1 has a delta arrangement of R, G and B color pixels as depicted in FIG. 64. By suitably driving the pixel shift elements 4-1 and 4-2, the image position to be seen by a user is shifted over two pixel pitches in the horizontal direction. Then, the number of effective image pixels can be increased by three times. It should be noted that in FIG. 63, a light ray emanating from a center of the image displaying LC panel 1 has three pixel shift positions 1, 2 and 3 which are shifted vertically for the sake of clarity.
In case of carrying out the pixel shift as explained above, the image information has to be changed or rewritten for a time interval during which an ordinary image of one frame is displayed without the pixel shift. Therefore, it is advantageous to use, as the image displaying LC panel 1, ferroelectric liquid crystal panel (FLC) and anti-ferroelectric liquid crystal panel (AFLC) which can have a higher response speed than other LC panels.
These LC panels having a high response speed are generally driven in a simple matrix mode, and thus a time period for rewriting image data of one frame is equal to a product of the number of scanning lines and a response time. Therefore, when the number of scanning lines of the image displaying LC panel is increased or the number of times of the pixel shift operations in a frame period becomes large, a relatively long time is required for rewriting the image data of one frame and the desired pixel shift could be no more attained and the high resolution could never be achieved.
For instance, in FIG. 63, when the NTSC color television signal is displayed on the AFLC with 525 scanning lines and response time per line of 20 .mu.s by effecting the pixel shift by two times for a frame period, a time x required for rewriting the image data of one frame will amount to EQU X=20 .mu.s.times.(525+525+525)=31.5 ms
Then, it is impossible to rewrite the image data during one frame period of 1/60 seconds, i.e. about 16.6 ms.
This problem is not specific to the image display on the liquid crystal panel, but also occurs for plasma display, electro-luminescent panel, photochromic panel and others having pixels arranged in matrix or in an image display apparatus driven by any driving mode other than the simple matrix mode.